Fundamental pitch frequency signal extraction system for complex signals



United States Patent [72] Inventor David E. Wood 3,327,058 6/1967 Coker179/1 Schenectady, N.Y. 2,938,079 5/1960 Flanagan 179/1 [21] Appl. No.636,390 2,891,111 6/1959 Flanagan 179/1 [22] Filed May 5, 1967 3,364,4258/1963 Peterson... 324/77 [45] Patented Dec. 22,1970 2,593,694 4/1952Peterson 175/183 l Asslgnee fc ggziffiz rzg sg gfi Pnrnary Exammer-Kathleen H. Claffy Assistant Examiner-Tom DAmico Attorneys-Richard R.Brainard, Marvin Snyder, Paul A.

Frank, Neuhauser, Melvin Goldenberg and EXTRACTION SYSTEM FOR COMPLEXSIGNALS Oscar adde" 14 Claims, 11 Drawing Figs.

[52] US. Cl 3127291; ABSTRACT: A System for extracting the fundamentalpitch fre uency from a full-wave rectified complex voice fre uenc 1 Cl010] 1/04 q q y [51] I3. I signal in p i Separatlng the Signal into aSpectral d Fleld of Search of its constituent Sinusoidal frequencies andrepetitively scanning the spectral band starting from the low frequencyd to detect the frequency of the first-occurring peak of rela- [5 6]References Cited tlvely large amplitude in each scan of the spectralband. Con- UNITED STATES PATENTS sistent detection of the same frequencyfor this peak in each of 3,450,989 6/1969 Dickinson the scansestablishes the fundamental pitch frequency signal, ,2 1963 PPQI-Wallowing generation of a signal of amplitude corresponding to 3,381,0914/ 1968 Sondhr 179/1 h f d mental pitch frequency.

Input from Rose! Signal Frequency Spectrum from Frequency Pea/c Detectorl5 Analyzer l2 ,'l6

i l Sunroof/1 1 46 Generator I Input from m, Hall 8 Fre uency M Bistopic I :I m Analyzer ii gfj z I Mu/lwbraiar a'ffarlanfmra 2%,

I 4/ 42 43 g I Narrow Sample I Ou/pul to I Track and nner/ring VOCALPITCH 05750701? f" e /8 L i .4

Output f0 Voicing Decision Logic /7 PQQQ,

input Log 'Log Log Log Log Lag Amplitude Amplitude Amplitude AmplitudeAmplitude Amplitude PATENIEU uc22 I370 8HEEI1UF3 35493306 Fuii aveFrequency Fm Rectifier Analyzer I ,0 Frequency l5 ,5pectrum PeakDetector weal Pitch Detector Fundamental 5% Pitci1 Frequency OutputSignal Fig Voicing Decision Anplitude Ratio Detector Voicing wind/cationSignal Absolute Level Tires/told Detector /4 Scan Scan 2 Scan 3 I Scan 4$can s I A v r Sc 6 M inventor:

i ANAL r250 vo /cs0 SPEECH His Attom y.

David E. Wood A PA ENTEUUECZZIQYU I saw 3 UF 3 3549305 Fundamental PitchFirs Frequency Harman/c g time and/ or i frequency g Threshold Level klnput from 4B 0 k fifl7g Voicing Decision Logic /7 I J{ e2 Hg. r" F 0 II i t/me lNg/gT I I l I Input from I l Fundamental E I Vocal Pitch PitchFrequency F 46 0 l a, Detector I6 60 Output S/gnaI me 1 Samoa/@719 v 114 Fi .45. k

g 0 qfime F /g. 7.

Output to Smoothing 7 Circuit /8 5 W x r 52 53 I FromVoca/ 5 PitchDetector l6 P Q 5 5 F//p-F/o 54 t I Am litude N07 From Amplitude p AND55 I Voicing 7hreshold Y dcotion Rot/a 09/9610! /3 Detector s fIns/(gnu,

l iom Absolute Level I rresho/d Detec 0/ l l VO/C/NG DECISION L06/C Fig.6

Inventor FUNDAMENTAL PI'ICI-I FREQUENCY SIGNAL "EXTRACTION SYSTEM FORCOMPLEX SIGNALS BACKGROUND OF THE INVENTION This invention relates topitch frequency signal extraction, and more particularly to extractionof the fundamental pitch frequency signal from a complex voice frequencysignal.

In speaking, voiced sounds are produced by air being forced out of thelungs and through the vocal tract. The air stream, in passing throughthe vocal cords, is modulated so as to produce a spectrum rich inharmonics. The size and shape of the throat and oral cavities ascontrolled by tongue and lip positions produce further modifications tothis air stream by reinforcing or attenuating particular portions of thespectrum. The final output energy is a function of the originalexcitation energy and resonance filtering in the cavities. The basicpulsation rate of the air passing between the vocal cords is present inthe output energy waveform as a series of repetitive complexes whichrepeat at this basic rate. The fundamental periodicity for vocal pitchof the output energy, which may be termed the fundamental pitchfrequency, varies from about 65 to 500 impulses per second for differentspeakers. Correct reproduction of the vocal pitch has been found to beessential to the synthesis of natural sounding speech, as required invocoder communication systems for example.

Accurate automatic measurement of vocal pitch has heretofore proven tobe a difficult problem. This problem stems from the wide acousticvariations which exist between different phonemes and theircombinations. By utilization of the instant invention however, thefundamental pitch frequency signal may be readily extracted with a highdegree of accuracy and minimum delay since, as I have discovered, a highamplitude peak occurs at the fundamental pitch frequency. Extraction ofthe fundamental pitch frequency signal is further facilitated byemployment of a high signal sampling rate which enables detection oftransitions from one phoneme to the next, as well as detection ofunvoiced periods.

SUMMARY OF THE INVENTION One object of this invention is to provide amethod and apparatus for rapidly and accurately determining thefundamental pitch frequency of a complex voice frequency signal fromwhich the fundamental pitch frequency signal may have been lost.

Another object is to provide a method and apparatus for reliablyobtaining the fundamental pitch frequency signal of a voice frequencysignal and for maintaining the signal constant during portions of voicedintervals in which tracking of the fundamental pitch frequency isdisrupted. Another object is to provide a simplified system for realtimedetermination of the fundamental pitch frequency signal of a complexvoice frequency signal without computer assistance, even in the presenceof signal disturbances.

Briefly, in accordance with a preferred embodiment of the invention, amethod of extracting the fundamental pitch frequency of a complex voicefrequency signal is described This method comprises the steps ofseparating the signal into a spectral band of its constituent sinusoidalfrequencies, and repetitively detecting the lowest frequency in eachband at which a relatively large amplitude peak occurs. Consistentdetection of the same frequency at which this lowest frequency,relatively large amplitude peak occurs establishes this frequency as thefundamental pitch frequency of the signal. Interpolation between abruptamplitude changes from one lowest frequency peak to the next enablesreconstruction of the fundamental pitch frequency signal.

,-,. According to another preferred embodiment of the invention,apparatus for extracting the fundamental pitch frequency signal of acomplex voice frequency signal is provided. This apparatus includesfrequency analyzer means responsive to the-voice frequency signal forseparating the voice frequency signal into a spectral bank of thesinusoidal signal constituents thereof, and signal amplitude peakdetecting means responsive to the frequency analyzer means and producinga pulse at the lowest frequency at which a relatively large amplitudepeak occurs in each spectral band produced by the analyzer means. Gatingmeans responsive to the amplitude peak detecting means are provided forpassing pulses produced by the amplitude peak detecting means to theoutput of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention believedto be novel are set forth with particularity in the appended claims. Theinvention itself, however, both as to organization and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of the complex signal fundamental pitchfrequency extractor of the instant invention;

FIG. 2 is a series of waveforms of analyzed voiced speech as produced bythe frequency analyzer of FIG. 1;

FIG. 3 is a block diagram of the frequency spectrum peak detector ofFIG. I;

FIGS. 4A4E are a series of waveforms to aid in the description of FIG.3;

FIG. 5 is a block diagram of the vocal pitch detector of the system ofFIG. 1;

FIG. 6 is a block diagram of the voicing decision logic circuitry usedin the system in FIG. I; and FIG. 7 is a schematic diagram of thesmoothing circuit of the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates the basicsystem for extracting the fundamental pitch frequency signal from acomplex voice frequency signal. Electrical signals corresponding tospeech and produced, for example, by a microphone or other type oftransducer are supplied through a high pass filter 10 to a full waverectifier 11. The function of high pass filter I0 is to block lowfrequency signal components that might otherwise interfere with thefundamental pitch frequency. The purpose of full wave rectifier 11 is toproduce a signal that has a strong frequency component at thefundamental pitch frequency during voiced speed; that is, to regeneratethe fundamental pitch frequency. This is essential since not only domany forms of telephone communications greatly attenuate or evencompletely block the fundamental pitch frequency, but high pass filter10 itself may remove the fundamental pitch frequency from the signal.

Output of full wave rectifier 11 is supplied to a frequency analyzer 12,a first input of an amplitude ratio detector 13, and an absolute levelthreshold detector 14. Frequency analyzer 12 may typically be that shownand described in D. E. Wood U.S. Pat. No. 3,243,703,issued Mar. 29, 1966and assigned to the instant assignee. The frequency analyzerrepetitively scans an input signal at a fixed repetition rate andproduces an output signal corresponding to the amplitude of thesinusoidal constituents or components of the input signal for each scan.Thus, the output signal of the frequency analyzer corresponds to asequence of scans, each scan ranging from one end of the scannedfrequency spectrum to the other. This is illustrated in the analyzedvoice speech waveforms shown in FIG. 2 wherein six scans, obtainedsequentially according to their numerical designation, are depictedgraphically with coordinates of logarithmic amplitude versus frequency.These waveforms indicate that, in each scan, a maximum peak of amplitudeis generally obtained at the fundamental pitch frequency, with lesserpeaks occurring at harmonics of the fundamental pitch frequency. Hence,the frequency analyzer can facilitate extraction of the fundamentalpitch frequency provided this peak at the fundamental pitch frequencycan be detected in each scan of the frequency analyzer by additionalapparatus in the system of FIG. I.

Frequency analysis of the rectified speech is typically performed byfrequency analyzer 12 over the range of 65 Hz to 500 Hz. The fundamentalpitch component for both male and female speakers falls within thisrange, so that it can be resolved and accurately defined. Prior toanalysis of the input signal, the signal is heterodyned within thefrequency analyzer to an intermediate frequency range. Because thefrequency analyzer typically includes a bank of i6 filters of 30 Hzbandwidth with a scanning repetition rate, in order of frequency throughthe filter bank, of 500 scans per second, each filter is sampled every 2milliseconds.

Output of frequency analyzer 12 is supplied to a frequency spectrum peakdetector 15 and a vocal pitch detector 16. Since the output offrequency. analyzer 12 is a continuous function of logarithmic amplitudeversus frequency for each scan, the frequency of the fundamentalsinusoidal component can be measured by detecting the lowest frequencyin the spectrum scan where energy is a maximum, as indicated in FIG. 2by the peaks occurring at the fundamental pitch frequency. Frequencyspectrum peak detector 15 detects high amplitude peaks by combiningdetection of zero amplitude slope with detection of a previouslypositive amplitude slope, so as to distinguish maxima from minima, andfurnishes a gating signal to vocal pitch detector 16 upon detection ofthe lowest frequency in each spectrum scan at which such maximum occurs.

Pitch detection is accomplished in vocal pitch detector 16 by detecting,once voicing is established, the vocal pitch fundamental frequency. Thisfrequency is indicated by the position of the lowest frequency highamplitude peak in the spectrum produced by analyzer 12. During theinitial buildup of energy at the onset of voicing, the position of thelowest frequency high amplitude peak shifts erratically, and oftenappreciably, from one scan to the next. As voicing is established,however, the position of the lowest frequency high amplitude peakbecomes relatively stable, and changes position only at rates resultingfrom changes in articulation. Thus, vocal pitch can be detected bymeasuring the position of the first high amplitude spectrum peak onsuccessive scans; when the frequency change from scan to scan becomessufficiently small, the frequency at which this spectrum peak occurs isthe vocal pitch fundamental frequency.

The vocal pitch detector is designed to automatically hold the lastfundamental pitch frequency value in the event the apparent pitch outputis violently disturbed by bursts of interference or oddities of speech.causing it to change more than a specified amount from the previousscan. A 2 millisecond interval between frequency scans is sufficientlyshort to allow smooth tracking of true pitch changes, while the holdprovision ensures continuity of output during the period of voicing.During the hold condition, the vocal pitch detector searches for a newvalue of pitch; if a stable component is located and voicing continuesto be indicated, the system effects a smooth transition from the heldvalue of pitch to the new value, even though the transition is madeabruptly by the vocal pitch detector.

Output signals from frequency spectrum peak detector 15, in addition tobeing supplied to vocal pitch detector 16, are also furnished to asecond input of amplitude ratio detector 13. Thus, ratio detector 13responds to the ratio of the logarithm of the maximum peak amplitude inthe rectified speech frequency spectrum to the rectified speechamplitude averaged over a period, which typically may be equal to 3 to 6times the scan times the scan time offrequency analyzer 12, or about 6to 12 milliseconds. This averaging period is suffciently long to avoidundue response to spurious transients in the signal. The periodicamplitude modulation of voiced speech varies from at least 10 percent tonearly lOO percent of the signal, and appears in the rectified speechspectrum as a strong component. Unvoiced speech or noise exhibits nosuch strong frequency component. Thus, a considerable margin exists inthe values of this ratio between voiced and unvoiced speech, with thehigher ratios indicating voiced speech.

Output of amplitude ratio detector 13 is supplied to a first input ofvoicing decision logic circuitry 17, furnishing a parameter upon which adecision as to whether or not voicing is present may be based. A secondinput to voicing decision logic 17 is supplied by absolute levelthreshold detector 14, which determines whether there is sufficientenergy in the output of full wave rectifier 11 to validly assume thatspeech is actually occurring; unless there is sufficient energy in thespeech input signal, there is no reason to operate the system. When allconditions for voicing are met at an acceptable confidence level, anoutput signal is produced indicative of presence of voicing.

A third input to voicing decision logic 17 is energized by outputsignals from vocal pitch detector 16 which indicate that stable trackingof the fundamental pitch frequency is occurring. Accordingly, anadditional output signal from voicing decision logic circuitry 17 issupplied to a smoothing circuit i8, enabling the smoothing circuit toproduce a fundamental pitch frequency output signal in response tosignals from vocal pitch detector 16 during periods beginning when bothvoicing is present and stable tracking of the fundamental pitchfrequency is taking place, and ending only when voicing disaptears.

The output signal supplied by vocal pitch detector 16 to smoothingcircuit 18 comprises a voltage with stepwise amplitude changes from onescan to the next. This voltage must represent a value corresponding to astable fundamental pitch frequency, as determined by voicing decisionlogic circuitry i7, before the voicing decision logic circuitry cansupply an output signal to smoothing circuit 18. When the fundamentalpitch frequency is stable and voicing is present, smoothing circuit l8minimizes the effect of voltage steps in the output signal of pitchdetector 16. This minimization continues until voicing next disappears.Since these steps are relatively closely spaced, such as at 2millisecond intervals, it is preferable to use a short smoothing timeconstant of approximately twice this spacing, or 4-5 milliseconds. Thus,relatively little delay is added by the smoothing operation. When novoicing is detected, the system output signal is held constant at itsvalue occurring at the instant voicing last disappeared; when stablevoicing conditions are detected, the system output signal comprises thefundamental pitch frequency output signal. The end of voicing causes thefundamental pitch frequency output signal to remain at a constantamplitude.

To briefly recapitulate, the system of FIG. 1 produces an output voltagecorresponding to the fundamental pitch frequency signal, during voicing.The speech input signal is furnished to full wave rectifier 11 throughhigh pass filter 10 which eliminates unwanted low frequency noise. Thefull wave rectifier reinserts the fundamental pitch frequency in theevent this frequency has been lost either in transmission or byattenuation in high pass filter 10. The output of full wave rectifier 11is broken into its constituent frequencies by frequency analyzer 12,which repetitively supplies scanned spectrums of frequency to vocalpitch detector 16 and frequency spectrum peak detector 15. Frequencyspectrum peak detector 15 supplies a gating signal to vocal pitchdetector 16. Thus, assuming voicing is present, the first peak in eachscan produced by frequency analyzer l2 and detected by frequencyspectrum peak detector 15 is supplied by vocal pitch detector 16 tosmoothing circuit 18. Moreover, when voicing is present, smoothingcircuit it? adds interpolation between the individual peaks furnished byvocal pitch detector 16 so as to provide a smoothly varying outputsignal of amplitude corresponding to the frequency of the fundamentalpitch frequency signal.

Whether or not voicing is present is determined by voicing decisionlogic circuitry 17, which requires both presence of sufficient speechsignal amplitude as determined by absolute level threshold detector 14,and indication of a sufficiently high amplitude lowest frequencyspectrum peak relative to the short term average of the rectified speechproduced by full wave rectifier ll, as determined by amplitude ratiodetector 13. Additionally, when an indication of stable tracking of theselected fundamental pitch frequency as determined by vocalpitch'detector 16 is also provided, albeit even momentarily, voicingdecision logic circuitry 17 produces a signal which al lows theamplitude of the fundamental pitch frequency, as

sensed by vocal pitch detector 16, to be supplied through smoothingcircuit 18 to the system output for as long as voicing continues.

FIG. 3 is a block diagram of frequency spectrum peak detector of FIG. 1.The peak detector comprises a differentiator 21 receiving input signalsfrom frequency analyzer 12 of FIG. 1. Output signals from differentiator21 are supplied to a hysteresis flip-flop 22. This hysteresis flip-flopcircuit, which may comprise a high gain differential amplifier, isactuated in the positive direction only when the positive input signalexceeds a threshold; however, this circuit returns to its initialcondition whenever the input signal returns to zero. A circuit of thistype is shown and described in D. E. Wood application Ser. No. 636,324filed concurrently herewith, now U.S. Pat. No. 3,522,545, and assignedto the instant assignee. Output signals from hysteresis flip-flopcircuit 22 are furnished to a second differentiator 23 which, in turn,provides an output signal to the input of an inverter circuit 24. Outputsignals from inverter circuit 24 are applied through a half waverectifier 25 to the input of vocal pitch detector 16 shown in FIG. 1.

Operation of the frequency spectrum peak detector of FIG. 3 may best beunderstood with the aid of the waveforms of FIGS. 4A 4E. Thus, each ofthe waveforms of FIGS. 4A4E represent voltage amplitude along a commontime abscissa. FIG. 4A represents the output signal of frequencyanalyzer 12 of FIG. 1 as supplied to the input of differentiator 21.This differentiator produces an output waveform as shown. in FIG. 43wherein zero output voltage corresponds to the zero slope at the voltagemaxima and minima in theoutput waveform of frequency analyzer 12.Although the waveform of FIG. 4A is plotted both against time andagainst frequency, the waveform of FIG. 413, as well as those of FIGS.4C-4E are plotted solely against time. For this reason, the waveform ofFIG. 4A also illustrates two major peaks corresponding to thefundamental pitch frequency and the first harmonic thereof.

Output of differentiator circuit 21 drives flip-flop circuit 22 into anON condition, whenever the amplitude of positive output voltage producedby the differentiator circuit exceeds a predetermined threshold level,as shown in FIG. 4B. When the output voltage produced by differentiator21 returns to zero, flip-flop circuit 22 returns to its OFF condition.The output pulses generated by flip-flop circuit 22, which areillustrated in FIG. 4C, are differentiated by differentiator 23, sothatupon initiation of an output pulse by flip-flop circuit 22 apositive spike is generated by differentiator 23; similarly, uponcompletion of each output pulse by flip-flop circuit 22, a negativespike is generated. The positive and negative spikes produced bydifferentiator 23, and which are illustrated in FIG..4D, are inverted byinverter circuit 24, and thereafter rectified by half wave rectifier 25,so that the output signal of peak detector 15 corresponds to positivevoltage spikes generated each time hysteresis flip-flop circuit 22switches to its OFF condition. The peak detector output signal isillustrated by the waveforms of FIG. 4E.

. FIG. 5 is a block diagram of vocal pitch detector 16, shown in FIG. 1.Input signals from frequency analyzer 12 of FIG. 1, which comprise therepetitive frequency spectrums produced thereby, are applied to theinput of an amplitude threshold detector circuit 40 which, in turn,passes signals above a predetermined threshold level to the signal inputof a gate circuitp4l. The control input of gate 41 is energized bysignals from frequency spectrum peak detector 15 of FIG. 1, so that thegate is opened at the instant the frequency spectrum peak has beendetected. The output voltage of the frequency analyzer at this instant,provided the amplitude thereof exceeds the threshold set by thresholddetector 41), is furnished to the input of a bistable multivibrator 42.Bistable multivibrator 42 is reset by a signal from the frequencyanalyzer each time the frequency analyzer is internally actuated to scana new spectrum of frequencies.

Rectangular output pulses produced by bistable multivibrator 42 aresupplied to a differentiator circuit 43 which, through a half waverectifier 39, provides narrow, sharp unipolar pulses to the inputs of asample and hold circuit 44 and a narrow track gate circuit 45. Thesenarrow, sharp pulses function as a sampling signal for sample and holdcircuit 44 and as an input signal for narrow track gate circuit 45.

The reset signal for bistable multivibrator 42, which originates infrequency analyzer 12 and is preferably the scan sync signal as producedby the Nth cycle amplifier illustrated in the aforementioned U.S. Pat.No. 3,243,703, is also supplied to the control or sample input of asecond sample and hold circuit 48, as well as to the input of a sawtoothgenerator circuit 46. This sync signal initiates a new sawtooth voltagewaveform each time it is supplied to the input of generator 46. Thesawtooth voltage wave is supplied to the signal input of sample and holdcircuit 44, as well as to one input of an amplitude comparator circuit49 which produces a narrow output pulse whenever the amplitude levels ofapplied input voltages are equal. The sawtooth voltage wave is alsofurnished to the signal input of a third sample and hold circuit 47.

Output signals from sample and hold circuit 44 are provided to thesignal input of sample and hold circuit 48. Output signals from sampleand hold circuit 48 are furnished to the second input to amplitudecomparator circuit 49 which, in turn, supplies output pulses to thecontrol input of narrow track gate circuit 45. Output of narrow trackgate circuit 45 is coupled to the control or sampling input of sampleand hold circuit 47, as well as to voicing decision logic circuitry 17shown in FIG. 1. Output signals from sample and hold circuit 47 aresupplied to the input of smoothing circuit 18 of FIG. 1.

Narrow track gate circuit 45 functions as a coincidence detector, andprovides an output signal corresponding to the pulses supplied theretofrom differentiator circuit 43 through half wave rectifier 39 as long asthese pulses substantially coincide in time with the pulses fromamplitude comparator circuit 49. Although gate 45 must be sufficientlywide in time to follow changes in vocal pitch frequency which producethe characteristic intonation of speech, these changes are relativelyslow in comparison with the frequency analyzer scan interval; hence,gate 45 generally prevents transients not connected with normal speechfrom appearing in the output signal thereof. In sample and hold circuit47, the output pulses from narrow track gate circuit 45 sample thesawtooth waveforms supplied from sawtooth generator 46. Thus, sample andhold circuit 47 provides a signal of amplitude corresponding to theamplitude of voltage produced by sawtooth generator 46 at the instant apulse is supplied'by narrow track gate circuit 45.

The vocal pitch detector functions by accepting each scan from thefrequency analyzer and passing the portion thereof of amplitude greaterthan a predetermined level through amplitude threshold detector 40 togate 41. If, at the instant a peak is produced by peak detector 15 theamplitude of the spectrum exceeds the level set by amplitude thresholddetec tor 40, thereby assuring that the detected peak is not merely alow level peak due to noise or some form of disturbance, gate 41 passesthis instantaneous spectrum amplitude to bistable multivibrator 42which, in turn, supplies a pulse to differentiator 43. Thedifferentiator thus supplies output pulses in the form of narrow spikeswhile voicing occurs. A single spike occurs during each spectrum scan atthe instant the first spectrum peak is detected. These spikes arerendered unipolar by rectifier 39. Because bistable multivibrator 42 isreset by the frequency analyzer only once per scan, only one spike isproduced per scan.

Sawtooth generator 46, being synchronized with the frequency analyzer,produces repetitive sawtooth waves of duration equal to the scan plusthe interval between two successive scans. Upon occurrence of eachnarrow spike from rectifier 39, the sawtooth voltage amplitude at thatinstant is supplied by sample and hold circuit 44 to sample and holdcircuit 48 and, upon occurrence of the next reset signal from frequencyanalyzer 12, is supplied to amplitude comparator 49. Due to the shortdelay introduced by sample and hold circuit 48, which is comparable tobut a fraction of the duration of each spectrum, comparator 49 receivesa signal from sample and hold circuit 48 which represents frequency ofthe selected peak occurring in the preceding scan. Hence, in amplitudecomparator 49, output of sample and hold circuit 48. representing thesawtooth voltage amplitude at the instant of occurrence of the previousspike from rectifier 39, is compared with the presently occurringsawtooth and, when coincidence is detected, amplitude comparator 4)supplied a gate pulse to the control input of narrow track gate 45.Thus, the

pulse produced by amplitude comparator 19 occurs during the no outputpulse is produced by narrow track gate circuit 45.

Thus, output signal information from circuit 45, by indicating: stabletracking of the fundamental pitch frequency signal constitutes a validindicator of the presence of voicing, (11.25: hence is supplied to oneinput of voicing decision logic circuitry 17 as well as to the samplingor control input of sample and hold circuit 47.

Output of sample and hold circuit 67 constitutes a signal which may varyonly slowly since, if the lowest frequency spectrum peaks occur atapproximately regular times in suecessive scans, output pulses fromnarrow track gate circuit 435 occur at approximately regular times andhence sample the sawtooth voltage from generator 46 at approximatelyregular times. The sampled amplitude is thus approximately unchangedfrom sample to sample. In the event stable tracking ceases, asmanifested by anticoincidence of pulses supplied to narrow track gatecircuit 45, sample and hold circuit 47 continues to produce a steadysignal of amplitude equal to that provided at the time of the mostrecent output pulse from gate 45.

Voicing decision logic circuitry 17, shown in FIG. 6, comprises a firsttwo-input AND gate 50 responsive to the continuous output signals bothof absolute level threshold detector 14, and of amplitude ratio detectorl3 through an amplitude threshold detector 51. When both inputs of ANDgate 50 are fulfilled, the AND gate provides a voicing indication outputsignal. In addition, the voicing indication output signal is sup pliedto one input of a second two-input AND gate 52, the second input ofwhich is fulfilled by a continuous output signal supplied by anintegrator circuit 54 in response to spikes produced by narrow trackgate circuit 45 of vocal pitch detector 16. Integrator 54, which maycomprise the well-known i 1? circuit type, is preferably shunted by adiode 55 in order t: allow AND gate 52 to respond immediately uponinitiation of a train of output pulses from narrow track gate 45. Outputsignals of AND gate 52, which are in the form of continuous signals,actuate a flip-flop 53 to its set condition. Output signals fromflip-flop 52, when in the set condition, are supplied directly tosmoothing circuit 18. Flip-flop 53 is reset through a NOT circuit 56from the output ofAND gate 50.

Voicing decision logic circuitry 17 produces a voicing indication outputsignal provided AND gate 50 receives signals both from absolute levelthreshold detector circuit 14, which requires that the output signalfrom full wave rectifier 11 is of sufficient amplitude as to make itlikely that a voicing signal is being supplied to the system, and fromamplitude ratio detector 13, which requires that the ratio of peakamplitude in each spectrum to the average amplitude threshold detector51 so as to make it likely that the output signal from full waverectifier 11 is a voice signal and not merely large amplitude noise.

AND gate 52 combines recognition of a voicing indication output signal,as produced by AND gate 50, with a stable tracking indication suppliedfrom vocal pitch detector 16 through diode-shunted integrator [M inorder to provide a large time constant for the smoothing circuit. Thus,the logic supplied by AND gate 52 indicates that a peak at the samefrequency in each spectrum is being detected, so that the fundamentalpitch frequency corresponds to the frequency at which this peak isconsistently detected. As long as AND gate 52 provides an output signal,an output is provided to smoothing circuit 18 from flip-flop circuit 53.Upon completion of the output signal from AND gate 5'2 however,smoothing circuit 18 continues to receive a signal from flipflop 53 foras long as AND gate continues to produce an output voicing indicationsignal, so as to maintain the large time constant in smoothing circuit18 for as long as voicing continues, even if stable tracking of thefundamental pitch frequency is no longer indicated by virtue of absenceof pulses from narrow track gate 45 of the vocal pitch detector.However, in the event the voicing indication is lost, NOT circuit 56resets flip-flop 53. The flip-flop thus deenergizes smoothing circuit18, removing the large constant thereof so as to allow the smoothingcircuit to quickly change output voltage values while the system seeks anew spectrum peak. During the time a new peak is being sought however,sample and hold circuit 47 maintains the fundamental pitch frequencyoutput signal constant. Once stable tracking and voicing have both beenreestablished, the smoothing circuit again receives its long timeconstant almost instantaneously, due to passage of the first outputpulse from narrow track gate circuit through diode 55 to the set inputof flip-flop circuit 53.

FIG. 7 shows the circuitry of smoothing circuit 18 of FIG. I. Thiscircuitry comprises a series-connected resistance 60 and capacitance 61which, together, function as an integrator with a time constantpreferably of about two spectrum periods. An INHIBIT gate 62; isconnected in parallel with resistance 60, and is controlled by outputsignals from flip-flop 53 in voicing decision logic circuitry 17. Thus,in absence of output signals from AND gate 52, flip-flop 53 initiallyproduces no output signal, so that gate 62 forms a short circuit acrossresistance 6t). Input signals which may be received from sample and holdcircuit 47 of vocal pitch detector 16 are thus passed directly throughgate 62 to the output of smoothing circuit 18. Under these conditions,the fundamental pitch frequency output signal as produced by smoothingcircuit 18 corresponds to what might be described as a stepwise or boxcar signal; that is, the signal is substantially a replica of the outputsignal produced by sample and hold circuit 47 of the vocal pitchdetector. On the other hand, when an output signal is produced by ANDgate 52 of voicing decision logic circuitry 17, a signal is provided atthe output offlip-flop 53 which inhibits gate 62', that is, the paththrough the gate is open-circuited. The effect of resistance 60 is thusinserted into smoothing circuit 18, establishing the large time constanttherein. The fundamental pitch frequency output signal therefore changesvery slowly from this time on, until the circuit of gate 62 is once moreclosed. By removing the large time constant during intervals in whichvoicing ceases, the output signal produced by smoothing circuit 18 canchange very rapidly, so that once voicing has again been detected by thesystem, along with stable tracking of the fundamental pitch frequency,the fundamental pitch frequency output voltage reaches an amplitudevalue corresponding to that of the fundamental pitch frequency veryrapidly. This provides the system with a very fast response.

The foregoing describes a method and apparatus for rapidly andaccurately determining, with a high degree of reliability, thefundamental pitch frequency signal of a complex voice frequency signalfrom which the fundamental pitch frequency signal may have been lost.The system further enables real time determination of the fundamentalpitch frequency signal of a complex voice frequency signal withoutcomputer assistance, even in the presence of signal disturbances.

While only certain preferred features of the invention have been shownby way of illustration, many modifications and changes will occur tothose skilled in the art. It is, therefore, to be understood that theappended claims are intended to cover all such modifications and changesas fall within the true spirit and scope of the invention.

1 claim:

1. A method of extracting the fundamental pitch frequency of a complexvoice frequency signal comprising the steps of periodically separatingsaid voice frequency signal into a spectrum of its constituentsinusoidal frequencies, repeatedly detecting the frequency of the lowestfrequency high amplitude peak in each frequency spectrum of the signal,comparing the frequency of the lowest frequency high amplitude peak in afirst spectrum with the frequency of the lowest frequency high amplitudepeak in a succeeding spectrum, and equating the fundamental pitchfrequency of said complex voice frequency signal to the frequency of thelowest frequency high amplitude peak of the spectrum upon detection ofthe lowest frequency high amplitude peaks at substantially identicalfrequencies in the compared spectrums.

2. The method of extracting the fundamental pitch frequency of a complexvoice frequency signal of claim 1 including the preliminary step offull-wave rectifying said voice frequency signal prior to separatingsaid voice frequency signal into a spectrum of constituent sinusoidalfrequencies.

3. The method of extracting the fundamental pitch frequency signal of acomplex voice frequency signal of claim 2 including high pass filteringsaid voice frequency signal prior to full wave rectifying said signal.

4. A method of extracting the fundamental pitch frequency signal of acomplex voice frequency signal comprising the steps of separating thevoice frequency signal into a spectrum of its constituent sinusoidalfrequencies, repeatedly detecting the lowest frequency high amplitudepeak in each frequency spectrum of the signal, storing a manifestationof the time at which said lowest frequency high amplitude peak occurs ina given spectrum, comparing the occurrence of the lowest frequency highamplitude peak of a first spectrum with the occurrence of the lowestfrequency high amplitude peak of a succeeding spectrum, and providing anoutput signal corresponding to an interpolation of said peaks as long assaid peaks occur at approximately the same time in each spectrum wherebysaid output signal represents a reconstructed version of the fundamentalpitch frequency signal.

5. The method of extracting the fundamental pitch frequency signal of acomplex voice frequency signal of claim 4 including the preliminary stepof full-wave rectifying said voice frequency signal prior to separatingsaid voice frequency signal into a spectrum of constituent sinusoidalfrequencies.

6. A method of extracting the fundamental pitch frequency signal of acomplex voice frequency signal comprising the steps of periodicallyseparating said voice frequency signal into spectra of its constituentsinusoidal frequencies, detecting the lowest frequency high amplitudepeak in each of said frequency spectra, storing a manifestation of thetime at which said lowest frequency high amplitude peak occurs in agiven spectrum, comparing the occurrence of the lowest frequency highamplitude peak of a first spectrum with the occurrence of the lowestfrequency high amplitude peak of a succeeding spectrum, detecting theamplitude of said voice frequency signal, and providing an output signalcorresponding to an interpolation of said peaks as long as said peaksoccur at approximately the same time in each of said spectra and theamplitude of said voice frequency signal exceeds a predetermined level.

7. The method of extracting the fundamental pitch frequency signal of acomplex voice frequency signal of claim 6 including continuing theoutput signal occurring upon last coincidence of said lowest frequencyhigh amplitude peaks of successive spectra upon cessation of a stablefrequency for successive lowest frequency peaks.

8. The method of extracting the fundamental pitch frequency signal of acomplex voice frequency signal of claim 6 wherein said step of detectingthe lowest frequency high amplitude peak in each frequency spectrum ofthe signal is performed only when said high amplitude peak is above apredetermined level with respect to the average amplitude of saidspectrum.

9. The method of extracting the fundamental pitch frequency signal of acomplex voice frequency signal of claim 7 wherein said step of detectingthe lowest frequency high amplitude peak in each frequency spectrum ofthe signal is performed only when said high amplitude peak is above apredetermined level with respect to the average amplitude of saidspectrum.

10. Apparatus for extracting the fundamental pitch frequency of acomplex voice frequency signal comprising: frequency analyzer meansrepetitively separating the signal into a spectrum of sinusoidal signalconstituents; peak detector means responsive to said frequency analyzermeans and producing a pulse at the lowest frequency at which a highamplitude peak occurs in each spectrum produced by said analyzer means;means for storing a manifestation of the time at which said lowestfrequency high amplitude pulse occurs in a given spectrum; means forcomparing the occurrence of the lowest frequency high amplitude pulse ofa first spectrum with the occurrence of the lowest frequency highamplitude peak of a succeeding spectrum; and gating means responsive tosaid comparison means for passing the output of each peak to the outputof said apparatus upon occurrence of said pulses at approximately thesame time in successive spectra.

11. The apparatus of claim 10 including smoothing circuit meansresponsive to said gating means for interpolating the output signalbetween successive ones of said peaks.

12. The apparatus of claim 11 including full-wave rectifier meanscoupling said voice frequency signal to said frequency analyzer means.

13. The apparatus of claim 10 including full-wave rectifier meanscoupling said voice frequency signal to said frequency analyzer means.

14. The apparatus of claim 13 including high pass filter means couplingsaid voice frequency signal to said full-wave rectifier means.

